This invention is directed to jet aircraft and more particularly to upper surface blowing jet aircraft.
In recent years attention has been directed to creating both military and commercial jet aircraft that have the capability of taking off and landing in short distances. It has been found that it is difficult to obtain the desired short take-off and landing benefits with conventional under-the-wing engine/airplane configurations. Thus, attention has been directed to other methods of and apparatus for obtaining the desired results. One such method and apparatus utilizes augmentor wings and a directed air flow to augment the lifting capability of the wings. Another approach has been to utilize upper surface blowing techniques. This invention is directed toward aircraft using upper surface blowing techniques.
An upper surface blowing aircraft is an aircraft wherein jet engines are mounted above and forward of the longitudinal axis of the wings so that the jet exhaust travels across the upper surface of the wings and flaps, forming the trailing edge of the wings. This arrangement is contrary to more classical jet engine/wing arrangements wherein the engines are "hung" beneath the wings. During take-off and landing, i.e., low speed operation, the jet exhaust of an upper surface blowing aircraft attaches to the upper surface of the associated wing and flap, the flaps being in an extended state. The attached airflow creates a downward thrust component, and a slightly rearward thrust component, the magnitude of the components depending upon the actual extended state of the flaps and the degree of jet exhaust turning. The downward thrust component of the jet exhaust augments the normal aerodynamic lift provided by the wings and, thus, allows an upper surface blowing aircraft to take off and land in relatively short instances. During high speed (cruise) operation, the flaps of an upper surface blowing aircraft are retracted and little or no flow attachment to the wings and retracted flaps occurs. Thus, during cruise the jet exhaust flows directly rearwardly, but still over the wing and flap surfaces.
One of the difficulties with upper surface blowing type aircraft relates to the attachment of the jet exhaust to the wing and flap surfaces during low speed operation. More specifically, the jet exhaust attachment occurs in accordance with what is known as the Coanda effect. The Coanda effect is defined as the tendency of a gas or fluid stream to follow a curved surface. This effect naturally occurs if the radius of curvature, the thickness of the stream and the negative pressure normally occurring on the side of the stream adjacent to the curved surface fall within certain interrelated boundaries. That is, the thickness of the stream that will turn about a predetermined radius of curvature is determined by the amount of negative pressure normally occurring on the side of the stream adjacent to the surface forming the radius of curvature. If all of the parameters do not fall within their interrelated boundaries the stream detaches or separates from the wing and flap surface. The problem, thus, is insuring that attachment takes place when desired because if the jet exhaust stream does not attach or becomes partially or fully detached the beneficial aspects of the Coanda effect are partially or fully lost. When the Coanda effect is lost, both the lift due to jet deflection and the increased aerodynamic lift of the aircraft due to upper surface blowing is decreased.
Therefore, it is the object of this invention to provide a method of enhancing the attachment of a jet stream to a wing and flap upper surface and increasing the jet stream turning effectiveness.
It is also an object of this invention to provide a method of enhancing the attachment of the jet exhaust generated by a jet engine mounted above the wing of an aircraft to the wing and associated flap upper surface of the aircraft.
It is also an object of this invention to provide a method of and an apparatus for improving Coanda flow attachment to the wing and flap surface of an upper surface blowing aircraft.